This invention relates to a method and apparatus for making wood and/or paper products from raw wood. More particularly, this invention relates to a novel arrangement for heating the drying gas that is passed through the dryer used in the course of making plywood veneer, particle board and other wood and/or paper products from raw wood.
Veneer is a thin sheet of wood of uniform thickness produced by peeling, slicing or sawing logs, bolts or flitches. Most wood species require steaming or heating before they can be cut into veneer. Veneer is used mainly to make plywood, a multilayered wood product in which the grains of adjacent layers are at right angles to each other. Before being made into plywood, veneer is usually dried to uniform moisture levels.
Particle board is a well known type of wood product consisting of wood fiber particles of varying sizes pressed or extruded as sheets and bound by a resin such as ureaformaldehyde. In the process of making particle board, the raw wood is usually steam heated and then dried to the necessary moisture levels. When faced with plywood, particle board is used extensively as a core material in furniture and cabinetwork and in the building industry for interior and exterior paneling.
In the processes for making veneer, particle board and other types of wood and/or paper products, the steam needed for treating the wood is obtained from a boiler and the drying of the wood is normally done in a dryer in which a drying gas, usually air, heated to a temperature of above 200.degree. F. (about 400.degree. F. for veneer, about 800.degree. F. for particles, and about 600.degree. F. for fibre) by a gas or oil burner, is passed over the wood and carries away from the wood vaporized liquid. The fuel needed for the burner used to heat the drying gas to the proper temperature increases the overall cost of the final product. Furthermore, the combustion gases produced in burning the fuel for heating the drying gas are a source of pollution.